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S-Cool Revision Summary

S-Cool Revision Summary

Interphase

DNA replicates as mitosis.

Prophase I

Homologous chromosomes condense ( synapsis ) to form bivalents. The chromatids become coiled around each other. As the chromosomes pair up ( homologous ), the twisting produces tension, and sometimes sections of chromatid may break and exchange new partners with corresponding sections of different chromatids. These breakage points result in "cross-overs" or Chiasmata .

Metaphase I

The bivalents move to the equator of the cell. Which pair of chromosomes orientates to which pole is completely random (called random assortment .)

Anaphase I

The pairs of bivalents seperate into chromatid pairs, each pair of chromatid is pulled to a pole.

Note: Unlike mitosis, there is no division of the centromeres at this stage.

Telophase I

The pairs of chromatids reach their respective poles, the cell divides.

Prophase II

New spindle is formed and the centrioles have replicated. Nuclear membrane disintegrates.

Metaphase II

The pairs of chromatids line themselves up on the equator as in mitosis, with sister chromatids orientated toward opposite poles.

Anaphase II

The centromeres divide and the chromatids separate, migrating to opposite poles.

Telophase II

The cell divides. Nuclear membranes and nucleoli are reformed. The chromosomes uncoil and go into interphase. The daughter cells have half the number of chromosomes present in the original cell.

Inside the ovary there may develop one or more ovules . Each ovule begins life as a small projection into the cavity of the ovary. As it grows and develops it begins to bend but remains attached to the ovary wall by a placenta .

At the start, the ovule is a group of similar cells called the nucellus . As it develops, the mass of cells differentiates to form an inner and an outer integument, surrounding and protecting the nucellus within, but leaving a small opening called the micropyle .

At the centre of the ovule is an embryo sac containing the haploid egg cell (the female gamete ).

Each anther contains 4 pollen sacs . Many pollen grains develop inside each pollen sac. It begins with a mass of large pollen mother cells in each pollen sac. All are diploid .

In each pollen grain the wall thickens and forms an inner layer ( the intine ) and an often highly sculptured outer layer ( the exine ). The surface pattern is different on pollen grains from different species. When the pollen grains are mature, the anther dries out and splits open (a process called dehiscence ) and the pollen is released.

Many plants favour cross-pollination , so pollen must be transferred to the stigma of another plant if sexual reproduction is to take place. Some flowers rely of the wind to carry pollen grains others rely on insects.

Self-pollination is where the pollen is transferred to the stigmas of the same flower or the stigma of another flower on the same plant. Self-pollination is obviously more reliable, particularly if the nearest plant is not very close.

If the pollen grain lands on a compatible stigma, a pollen tube will grow so that eventually the egg cell, hidden away in the embryo sac, can be fertilised . A tube emerges from the grain, its growth being controlled by the tube nucleus at the tip of the tube. It may grow downwards in response to chemicals made by the ovary (a response known as chemotropism).

Like in plants it is the male gamete that needs to be transferred to the female gamete. The female gamete is fertilised and develops inside the mother's body so the reproductive systems of both males and females are highly adapted for this.